1. Field of the Invention
The invention relates to an image signal processing apparatus, image signal processing method, prediction coefficient generating apparatus, prediction coefficient generating method, and programs for causing a computer to execute the methods, which use orthogonal transformation for encoding. Specifically, the invention marks the degree of degradation of a block on the basis of a difference when re-encoded and an activity of the block to make it possible to appropriately remove distortion in accordance with the degree of degradation of the block.
2. Description of the Related Art
In recent years, a discrete cosine transform (DCT) is widely used in JPEG, MPEG, and the like, as encoding that uses orthogonal transformation. Distortion caused by the above encoding generally includes block distortion in which a step between adjacent blocks is apparent or mosquito noise that tends to occur around an edge. To remove such distortion, a low-pass filter is, for example, applied to portions that appear to be block distortion or mosquito noise to remove the distortion.
However, distortion removal may possibly be applied to an edge portion having no block distortion or a detail that is not mosquito noise, resulting in degradation of image quality. To solve the above problem, for example, Japanese Unexamined Patent Application Publication No. 9-331526 describes a technique that average value data of a target block and adjacent blocks are used as a class code to separate blocks containing an edge or a detail from blocks in which block distortion tends to occur. Thus, it is possible to execute processes suitable for the respective blocks. In addition, Japanese Unexamined Patent Application Publication No. 2006-121131 describes a technique that it is checked for an isolated coefficient value at a predetermined position of DCT coefficients to detect a mosquito noise portion.